Java Annotations - looking for an example of RetentionPolicy.CLASS

Question

according to Java Annotation API:

• RetentionPolicy.CLASS Annotations are to be recorded in the class file by the compiler but nee

Answer 1

Minimal example

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.CLASS)
@interface RetentionClass {}

@Retention(RetentionPolicy.RUNTIME)
@interface RetentionRuntime {}

public static void main(String[] args) {
    @RetentionClass
    class C {}
    assert C.class.getAnnotations().length == 0;

    @RetentionRuntime
    class D {}
    assert D.class.getAnnotations().length == 1;
}

If we use javap on the annotated classes, we see that the Retention.CLASS annotated class gets a RuntimeInvisible class attribute:

#14 = Utf8               LRetentionClass;
[...]
RuntimeInvisibleAnnotations:
  0: #14()

while Retention.RUNTIME annotation gets a RuntimeVisible class attribute:

#14 = Utf8               LRetentionRuntime;
[...]
RuntimeVisibleAnnotations:
  0: #14()

So the information is present on both cases in the bytecode.

Therefore, Runtime.CLASS can be used to associate arbitrary metadata to a class which bytecode manipulation tools can use, without interfering with runtime-visible behavior.

Examples on GitHub for you to play with.